Photoelasticity is the phenomenon of stress (or strain) induced birefringence in optically transparent materials. Many of those materials, in their unstressed state, are isotropic. When a uniaxial stress is applied to those materials, a difference in the index of refraction is introduced between the stress direction and directions orthogonal to it. Sensors based on this phenomenon have been reported previously by I. I. Slezinger et al. in U.S. Pat. Nos. 3,950,987 and 4,010,632. In addition, sensors based upon that phenomenon but incorporating fiber optic technology have been reported by W. H. Glenn and R. G. Tomlinson in U.S. Pat. Nos. 4,321,831 and 4,368,645. None of those patents, however, addresses the problem of making the sensor insensitive to changes in ambient conditions that produce unwanted changes in the sensor output.
It is known that stressing of a non-birefringent single mode optical fiber destroys mode degeneracy in the fiber and thereby produces birefringence. A strain measuring device using that property of the single mode optical fiber in its photoelastic sensing element is described in U.S. Pat. No. 4,173,412 granted to M. Ramsay and S. H. Wright. In that patent, an arrangement is described in which a single mode optical fiber is strained while polarized light having one component in and another component perpendicular to the strain direction propagates through the fiber. The two polarized components of the light emerging from the fiber are then used to obtain a measurement of the strain on the fiber. The Ramsay and Wright patent, like those cited previously, does not address the problem of making the photoelastic sensing element of the device insensitive to changes in temperature, air pressure, and other ambient conditions that produce unwanted changes in the sensor output. This specification addresses that problem and describes an arrangement of apparatus that solves the problem for single mode optical fiber photoelastic sensors through the use of optical common mode rejection.